Radio frequency identification (RFID) is a rapidly growing segment of today's automatic identification and data collection industry. RFID technology is considered a complement and likely an eventual replacement for bar code technology as RFID technology overcomes certain recognized bar code limitations. In particular, a visual line of sight is not required between a reader and a tagged RFID object, as RFID technology is not optically based. In a read mode, an RFID tag wirelessly receives data from a reader or interrogator to update or change data stored in the tag. In a write mode, the tag wirelessly transmits data to supply information stored in the tag to the reader.
RFID tags (also referred to as transducers) enjoy a wide variety of applications, including automatic fair collection on bridges, toll roads, and public transit vehicles; wireless pay-at-the-pump payment systems, hands-free access control security systems and identification and location of an object to which the tag is affixed. The tags provide a quick, flexible and reliable wireless system to detect, identify, track and therefore manage many different items. For example, supply chain logistics is expected to be the largest and fastest growing application for RFID tags that can operate over distances of about 20 feet. Affixing a tag to cases, cartons and pallets permits efficient identification of the contents of mixed pallets during material handling operations, such as truck loading or unloading. Incoming pallets at shipping and receiving facilities can be automatically routed to the proper destination by reading the RFID tags. In a warehouse environment, workers can scan storage areas using an RFID reader that automatically detects the item and its stored location.
RFID tags can be read through packaging, shipping containers, and most materials except metal. By interrogating a tag affixed to an object, including retail articles, materials, containers and enclosures, the object's location can be determined. For example, an RFID tag affixed to retail merchandise causes a reader to sound an alarm if a shopper attempts to exit the establishment without paying for the item.
Generally, an RFID tag comprises an integrated circuit in conductive communication with an antenna printed or etched on a dielectric substrate. RFID tags operate either passively (i.e., without an internal battery) or actively (i.e., powered by a separate battery). Information is stored in the integrated circuit and transmitted by the antenna over a radio frequency communications link to the external reader. Information is received over the link from the reader, for storage in or processing by the integrated circuit. Other tag embodiments only respond to interrogation signals from the reader. The response is typically digital, i.e., a one or a zero is transmitted as the response (where each response type is referred to as a modulation state). For certain embodiments, the lack of a response is interpreted as a zero response. Ones and zeroes can be transmitted by spoiling the impedance match between the integrated circuit and the tag antenna. In one state the antenna is shorted and in the other state the antenna load comprises the impedance of an active integrated circuit. Alternatively, an active type tag can switch a lossy element into the circuit as one of the two reply states for the reader to sense. A passive tag does not include a conventional transmitter, but instead comprises only a switched lossy element that generates the two states. However, in the description herein the tags are referred to as comprising generic receiving/transmitting elements, recognizing that the conventional mechanisms for receiving and transmitting radio frequency signals may not be employed. Tags vary in size as necessitated by the application, the operating frequency of the radio communications link and the processing and data storage capabilities of the integrated circuit. Information transmission speed and signal range depend on many factors, including operating frequency of the communications link, antenna size, tag output power, and signal noise and interference that affects the signal quality in both the transmitting and receiving modes.
Information is sent to and read from the RFID tag over the radio frequency communications link. In passive systems, which are the most common, an RFID reader transmits a radio frequency signal that “wakes up” the tag and provides power to the integrated circuit, enabling it to transmit data to the reader or store received data. An active system employs a separate battery to boost the effective operational range of the tag. Active tags may also periodically transmit a signal, without prompting, so that data may be captured by one or more readers located proximate the tag. For example, a plurality of fixed readers in a warehouse periodically receive tag signals for use in determining the tag location and thus the location of the product to which the tag is attached. The data received by the reader is typically sent from the reader to a host computer or processing system via a network interface. In certain applications, an encryption algorithm encrypts the information signal to ensure security and integrity of the data sent between the tag and the reader.
The reader may be integrated into a permanently-fixed reading station or disposed in a hand-held terminal. The fixed station is positioned at strategic points, such as a facility entrance/exit, dock door, or an assembly line.
A plethora of RFID tag technologies and specific tag designs are known. The application determines the appropriate tag technology, including operating frequency, system performance and tag memory capacity. Performance of passive RFID tags is determined primarily by the tag operating frequency. Low frequency RFID systems operate at about 1-5 kHz with a typical maximum read range of up to about 20 inches. High frequency systems operate at about 13.56 MHz with a typical maximum read range of up to about three feet. Ultra-high frequency RFID tags operate at multiple frequencies including 915 MHz and 2.45 GHz with a read range of up to about 20 feet.
The lower frequency tags are moderately resistant to shielding but exhibit limited functionality due to signal bandwidth constraints. In particular, systems based on these tags generally operate reliably only when a single tag is in an interrogation zone at any given time. Tags operating at these lower frequencies also tend to be relatively bulky and expensive to manufacture.
At the higher frequencies (13.56 MHz and above), the additional available bandwidth (up to about 30-40 MHz) allows the system to reliably and simultaneously process multiple tags in the interrogation zone. This feature is highly desirable for many tag applications.
The tag antenna geometry and radiating properties depend on the tag operating frequency. For example, RFID tags operable at about 2.45 GHz typically comprise a dipole antenna or a folded dipole antenna. At about 13.56 MHz, the RFID tag antenna preferably comprises a spiral antenna, a coil antenna or an inductively loaded dipole.
Irrespective of the specific antenna geometry and characteristics, the antenna receives an information signal from a tag interrogation source or reader. This signal commands the tag to perform one or more functions, including transmitting a reply signal to the reader, while also supplying energy to power the tag's integrated circuit. For optimal performance, the antenna characteristics must be matched to the tag integrated circuit and to the interrogation device.
For tags operating in the high MHz to GHz range, the most important antenna characteristic is antenna length. In most applications, the effective electrical length of the dipole antenna is a half wavelength or a multiple of a half wavelength of the information signal frequency. For tags operating in the low to mid MHz region where a half wavelength antenna is impractical due to tag size limitations, the important antenna characteristics include antenna inductance and the number of turns in the antenna coil.
All antenna geometries require good electrical conductivity. Typically, metals such as copper or aluminum are used, but other conductors, including magnetic metals such as permalloy, are also acceptable and are, in fact, preferred for some applications.
It is known that maximum energy is transferred between the integrated circuit and the antenna when the integrated circuit input impedance substantially matches the antenna impedance. In certain embodiments, the tag includes a tuning capacitor to provide the impedance match. The capacitor comprises a discrete component or an element of the antenna, i.e., the capacitor is integrated into the antenna structure. The capacitance value is selected so that when considered with the antenna inductance, the resonant frequency of the composite circuit, given by fres=1/(2*π(LC)1/2), where C=capacitance (in Farads) and L=inductance (in Henries) closely matches the desired operating frequency of the RFID tag system. Those skilled in the art recognize that an exact impedance match is seldom attainable and is not required, but the energy transfer improves as the impedances are more closely matched. Generally, an impedance match in the range of 10-20% is acceptable. In another embodiment, the capacitor comprises a distributed capacitor, which may be desired to reduce tag size, particularly tag thickness, and to minimize manual assembly efforts. In some tag designs, particularly tags operating at very high frequencies, such as 2.45 GHz, a tuning capacitor may not required.
It is known that performance of the tag antenna, and thus performance of the tag, depends on the electrical properties of the item to which the tag is attached, as electrical properties of both the item enclosure and its contents may affect tag performance. In particular, the dielectric constant of the enclosure material (e.g., cardboard, plastic, paper) and/or the contents can shift the antenna resonant frequency, degrading tag performance due to attenuation of the received or transmitted signal. Since the type of material to which the tag will be affixed is generally not known during tag design and development, the tag is typically designed and tested for free space operation. Later application of the tag to an item may cause the above-mentioned resonant frequency shift.
Empirical testing of a tag affixed to various types of material reveal that for most materials the antenna resonant frequency shifts downwardly from its free space value. To overcome the frequency shift it is possible, although costly and complex, to design and fabricate a unique antenna, and thus a unique tag, for each type of material. However, this solution requires production and inventory tracking of various tag models, each including an antenna optimized for operation with a specific material type.